1. Field of the Invention
The present invention relates to high pressure pumps, and more particularly to a poppet valve assembly incorporated in such pumps.
2. Background Art
There are various applications for high pressure pumps, one being to supply very high pressure fluid (e.g., water) so that this water may in turn be discharged in the form of a high velocity jet which can be used for cutting, abrading, etc. A common configuration for such a pump is to employ a reciprocating piston which operates in a high pressure cylinder to direct the fluid to a nozzle. On the pressure stroke, the plunger or other piston can generate pressures on the order of 25,000-40,000 PSI or more, while during the intake stroke the pressure in the cylinder drops to essentially nil.
An outlet check valve is positioned at the end of the pumping cylinder. Typical check valves used in this application include poppet valves, where the valve element has a transversely extending contact surface which fits against a matching contact surface of a valve seat. Poppet valves have generally proven satisfactory for these high pressure applications, but they exhibit a tendency to develop excessive wear, necessitating undesirable down time for repairs.
Before discussing the specifics of this problem, however, it is believed that a fuller appreciation will be gained by a review of a pump assembly having a conventional poppet valve of this type. FIG. 1 shows a system in which there is a pump assembly 10 having an end cap housing 16 mounted on the end of a cylinder housing. The cylinder housing is provided with a cylinder chamber in which the high pressure piston reciprocates. The poppet valve assembly 24 is mounted at the end of the cylinder chamber, and this is made up of a valve body 26 in which there is a central discharge passageway 28, and also several inlet passages 30 which are spaced radially therefrom. The discharge passageway 28 passes through poppet valve 32 into a high pressure valve chamber 34, and thence through an outlet opening 36 into an accumulator (shown schematically at 38). Typically, there are several such pumping assemblies which sequentially supply the accumulator 38 so as to sustain a constant supply of high pressure fluid; valve chamber 34 is thus constantly filled with very high pressure fluid, typically at 25-40,000 PSI or higher.
The poppet valve is made up of a cylindrical poppet element 60 mounted on the end of a guide 62 which reciprocates within a retaining cage 64. A compression spring 66 presses from an end wall 68 of the retaining cage against a shoulder 70 about the rear of the guide. The cage 64 is provided with openings 72 which permit fluid to flow around the poppet element 60 and out into chamber 34, and vents 73 are also provided to facilitate movement of the guide within the cage.
A cylindrical valve seat 74 is positioned within the base of cage 64. This has a central flow passage 75 which defines a continuous portion of outlet passageway 28. On the discharge stroke of the pumping cylinder, the pressure in passageway 28 lifts element 60 off seat 74, so that the fluid flows out into the valve chamber. Then, on the intake stroke, the pressure in passageway 28 drops off, and the poppet element 60 is forced into sealing abutment with seat 74 by compression spring 66 and the pressure in chamber 34; this seal prevents the high pressure fluid in the chamber from escaping back into the cylinder through passageway 28. The extreme pressure differential between chamber 34 and flow passage 75 during the intake stroke creates a very high interface stress between the poppet and seat, and this must be significantly higher than the fluid pressures to ensure that an effective metal-to-metal seal is established.
As noted above, rapid wear rates have been encountered in operation of these poppet valves. The wear has been observed to occur at the interface between the poppet element and the seat, where there is considerable erosion of material from both elements. Upon investigation, it has been found that this erosion stems not so much from the passage of fluid between these parts, but is instead mostly caused by relative movement between the element and seat. Specifically, the fluid in the chamber exerts a continuous compressive force against the exterior of the valve seat, this being communicated through a small annular space 78 between the seat and the socket 80 in which it is received, while the pressure in the central flow passage fluctuates with the strokes of the pumping piston. As a result, the annular wall of the seat is displaced inwardly by the pressure differential during the intake stroke, and then expands back outwardly during the discharge stroke. The poppet element, however, does not undergo the same cyclic deformation: as seen in FIG. 1, the conventional element 60 is a solid metallic component, and thus the pressure in chamber 34 is resisted by the non-fluctuating compressive strength of the solid body. As a result, considerable relative movement occurs between the mating surfaces of the seat and element while these are in engagement. This, combined with the high interface stress, leads to plastic deformation and fretting wear, which causes the valve to leak. Once the leaks begin, the wear rate accelerates and performance degenerates rapidly.
Accordingly, there exists a need for a poppet valve assembly for use with very high pressure reciprocating pumps which reduces or eliminates relative movement of the mating faces of the poppet element and valve seat while these are in engagement, so as to minimize wear during operation. Furthermore, there is a need for such an assembly which is economical to manufacture, and which is compatible with other components of existing types of such reciprocating pumps.